This invention relates to a voltage boost circuit and, in particular, to a boost circuit that limits starting current to a load, such as a piezoelectric driver.
A piezoelectric actuator requires high voltage, greater than typical battery voltages of 1.5 to 12.6 volts. A “high” voltage is 20-200 volts, with 100-120 volts currently being a typical drive voltage. Some line driven power supplies for actuators provide as much as 1000 volts. Producing high voltage from a battery is more difficult. As noted in U.S. Pat. No. 7,468,573 (Dai et al.), the high voltage required “to drive piezoelectric actuators in today's small electronic devices is undesirable.” The solution proposed in the '573 patent is to use two pulses of “lower” voltage instead of a single pulse at high voltage. The “lower” voltage is not disclosed. Single layer actuators generally require a higher voltage than multilayer actuators. Multilayer actuators have the advantage of providing greater feedback force than single layer actuators.
Thus, there is a need for a battery powered driver, that is, a single chip power supply, for piezoelectric devices. A voltage boost circuit can be used to convert the low voltage from a battery to a higher voltage for the driver. In a boost converter, the energy stored in an inductor is supplied to a capacitor as pulses of current at high voltage.
FIG. 1 is a schematic of a basic boost converter well known in the art; e.g. see U.S. Pat. No. 3,913,000 (Cardwell, Jr.) or U.S. Pat. No. 4,527,096 (Kindlmann). Inductor 11 and transistor 12 are connected in series between supply 13 and ground. When transistor 12 turns on (conducts), current flows through inductor 11, storing energy in the magnetic field generated by the inductor. Current through inductor 11 increases quickly, depending upon battery voltage, inductance, internal resistances, and the on-resistance of transistor 12. When transistor 12 shuts off, the magnetic field collapses at a rate determined by the turn-off characteristic of transistor 12. The rate of collapse is quite rapid, much more rapid than the rate at which the field increases. The voltage across inductor 11 is proportional to the rate at which the field collapses. Voltages of one hundred volts or more are possible. Thus, a low voltage is converted into a high voltage.
When transistor 12 shuts off, the voltage at junction 15 is substantially higher than the voltage on capacitor 14 and current flows through diode 16, which is forward biased. Each pulse of current charges capacitor 14 a little and the charge on the capacitor increases incrementally. At some point, the voltage on capacitor 14 will be greater than the supply voltage. Diode 16 prevents current from flowing to supply 13 from capacitor 14.
A problem with the converter shown in FIG. 1 is that, when capacitor 14 is not charged, the voltage across diode 16 is maximum and current is limited by the internal resistance of the inductor. Adding resistance to reduce current reduces the efficiency of the circuit during normal operation. A high current results in a high voltage that can damage piezoelectric or other devices powered by the converter. The high current also puts a significant load on the low voltage battery powering the boost circuit.
It is known in the art that pulse width, i.e. the period during which transistor 12 conducts, affects current (as long as the inductor does not saturate). Over the years, the circuit of FIG. 1 has been embellished with various feedback loops, some of which modulate pulse width; e.g., U.S. Pat. Nos. 7,106,036 (Collins) and 7,129,679 (Inaba et al.) The '679 patent discloses that gradually changing duty cycle during startup gradually increases the output voltage from the converter. The gradual change is accomplished by a closed loop feedback circuit that significantly increases the cost, complexity, and power consumption of the converter.
In view of the foregoing, it is therefore an object of the invention to provide a soft starting, high voltage driver for piezoelectric devices.
Another object of the invention is to minimize power drain by single chip, battery powered drivers.
A further object of the invention is to limit peak current in a boost converter, thereby preventing saturation of the inductor, minimizing power consumption, and avoiding damage to loads.
Another object of the invention is to provide a simple, soft start mechanism for a boost converter.
A further object of the invention is to provide an open loop, soft start converter.